A large number of pixels and a larger screen are sought for liquid crystal displays. In the past few years, the active matrix system using thin-film transistors (TFTs) has become dominant in order to achieve the demanded high image quality. In contrast to a liquid crystal display that requires a backlight, a self-emitting organic EL (also referred to as an organic light-emitting diode (OLED)) has advantages not present in liquid crystal displays and has been developed at a rapid pace over the past few years.
In the manufacture of TFT liquid crystal displays or organic EL displays, the so-called TFT array test is performed to electrically test whether or not the finished TFT array operates electrically in the stage where the TFT array was formed on the glass substrate, that is, before sealing the liquid crystal or the organic EL coating process. Testing is extremely important in improving the final yield of finished products in display manufacturing. In the TFT array test stage, if electrical defects were discovered in the TFT circuit driving specific pixels, the defect is repaired when the defect can be repaired, based on the TFT array test information. In addition, later processes can be stopped when there are many defects and a decision of defective product can be made in advance in the shipping inspection after assembly of the display. The advantage of some actions for this type of defective product is the omission of subsequent expensive processes such as bonding the color filters and sealing the liquid crystal for liquid crystal displays or performing the organic EL coating process for organic EL displays.
A TFT array usually forms a drive circuit for each pixel using a plurality of TFTs on a glass substrate of only the number corresponding to the number of pixels in the display. In the past few years, TFT arrays have generally been formed by using amorphous silicon or low-temperature polysilicon. FIG. 13 shows an example of a typical TFT drive circuit for one pixel in the popular liquid crystal display. In the figure, reference 250 denotes a data line; 251, a gate line; 252, a common line; 253, a liquid crystal; and 254, a transparent electrode using indium tin oxide (ITO). Usually, the structure formed on a glass substrate wherein the number of drive circuits, shown in FIG. 13, is only the number of pixels in a two-dimensional configuration is called a TFT array. Since an actual TFT array test for the liquid crystal is normally performed before sealing the liquid crystal 253, the drive circuit is electrically tested in the state without liquid crystal 253 in FIG. 13.
The number of exposed ITO electrodes 254 equal to only the number of pixels is arranged in a two-dimensional array on the surface of the glass substrate in the stage wherein the TFT array was formed. The testing method for this type of drive circuit usually electrically switches the TFTs and measures the potential to determine whether a normal potential is generated on the surface of the ITO electrode 254. In the state wherein a voltage was applied to the data line 250, the selected TFT transistor can be set in the ON state by applying a voltage to the gate line 251 of the drive circuit to be tested. At this time, the TFT transistor can be judged to be normal if the same voltage as the voltage applied to the data line is generated in the ITO electrode 254.
The proposed methods for measuring the electrical potential of the surface of the ITO are (1) a method that reads the charge stored temporarily in an electrostatic capacitor Cs in reverse through the data line, (2) a method that irradiates an electron beam on the ITO surface and measures the potential from the quantity of secondary electrons generated corresponding to the surface potential, and (3) a method that indirectly measures the potential as light information by using a nonlinear electro-optical effect such as the Pockels effect.
Conventionally, the quality of a drive circuit could be decided by measuring the surface potential of the ITO similar to the example of the drive circuit for liquid crystal shown in FIG. 13. On the other hand, since an organic EL display is a self-emitting display not requiring a backlight, the brightness of each pixel should be controllable, and the ability to control the drive current of each pixel is required. Therefore, a TFT array tester must measure the current drive capacity of the drive circuit from the electrode surface composed of ITO, for example, before coating the organic EL. Consequently, a TFT array tester for liquid crystal with the object of evaluating the characteristics of a constant voltage drive circuit in conventional use cannot be applied to the evaluation of organic EL displays.
To solve the above-mentioned problem, an inspection method using an additional conductive film for inspection (refer to Japanese Kokai Unexamined Patent Publication No. 2002-108,243) and an inspection method that introduces an electrolytic fluid between the pixel electrode of the TFT array and the opposing detection electrode (refer to Japanese Kokai Unexamined Patent Publication No. 2002-72198) were proposed. However, according to the former, the causes of defect generation increase because a conductive film for inspection is formed, this film must be removed after inspection, and additional processes are required. Therefore, the former method is not necessarily appropriate when the manufacturing yield is considered. According to the latter method, a substrate including the TFT array contributes to the causes of defect generation because parts that should not be submerged in electrolytic fluid are also included. Consequently, avoiding this kind of wet process is preferred. Furthermore, another method to consider is a method wherein an electromagnetic signal source is disposed on the back surface of the inspection electrode, and the air between a transparent inspection electrode and a pixel electrode is ionized by this electromagnetic signal so that an electromagnetic signal such as X-rays generated by the above-mentioned electromagnetic signal source passes through the inspection electrode, and current flows between the inspection electrode and the pixel electrode (refer to Japanese Kokai Unexamined Patent Publication No. 2002-123,190). However, in this structure, sufficient current density cannot be obtained when inspecting the TFT element operation even when the electrical conduction of the circuit can be inspected.
Therefore, an object of the present invention is to provide probe means capable of measuring the current drive capability of a current-driven TFT array such as for organic EL, in particular, before the organic EL coating process without physical contact with the ITO surface in a testing apparatus for testing the electrical characteristics of a TFT array formed on a display substrate. In addition, another object of the present invention is to provide a display substrate testing apparatus using the probe means.